People with the human immunodeficiency virus (HIV) have a surprisingly active thymus, an organ at the base of the neck that sends out immune cells but winds down with age. Researchers assumed that anyone with HIV would have an inactive thymus due to attack by the virus and normal ageing. But when Michael McCune of the University of California in San Francisco, USA, examined 99 adults with HIV, 47 had abundant thymus that may help the immune system rebound after antiviral therapy.